Download Global Cycle Analysis of N2O Using Isotopomers Sakae TOYODA

Global Cycle Analysis of N2O Using Isotopomers
Sakae TOYODA, Tokyo Institute of Technology
1. Introduction
Nitrous oxide (N2O) is one of the greenhouse gases in the troposphere (IPCC, 2007) and is the most
important ozone-depleting gas in the stratosphere (Ravishankara et al., 2009). Its global average
tropospheric concentration in 2010 is about 322 ppb, which is lower than that of carbon dioxide (CO2)
by three orders of magnitude. However, it has about 300 times greater global warming potential than
CO2 over a 100-year time scale, and is increasing at the rate of about 0.7 ppb yr-1. Sources of N2O
include natural and agricultural soils, aqueous environment such as oceans, rivers, and lakes,
industrial processes like fossil fuel combustion, biomass burning, animal and human wastes (IPCC,
2007). About 90% of these sources are related to microbiological processes such as nitrification and
denitrification, which occur naturally and can be enhanced in soils and waters that are enriched in
nitrogen species due to human activity. In spite of a number of studies based on concentration
analysis, there is still great uncertainty about the estimated magnitudes of global N2O sources. This is
mainly because microbial activity in soils or waters is sensitive to environmental factors and thus
production of N2O is not occurring uniformly with respect to space and time. In addition, it is difficult to
identify the microbial pathway that mainly contributes to the N2O production in each study site.
2. Stable isotopes as a tool to investigate global cycle of N2O
Although natural abundances of stable isotopes are almost constant (e.g.,
14
N and
15
N respectively
account for 99.64% and 0.36% of elemental nitrogen), they actually show a very small change during
physical or chemical processes because of small difference in their bond energy or kinetic energy
(isotope effect). Such a small variation is expressed by relative difference in isotope ratio (e.g.,
15
N/14N) between sample and reference material. In chemical reactions, isotope ratio (or isotopomer
ratios, when molecular species are considered) are determined by isotope (isotopomer) ratios in
precursors and isotope fractionation factor that is specific to each reaction. Therefore,
isotope/isotopomer ratios provide qualitative information that complements the quantitative
information obtained through mixing ratio analyses. We developed a mass spectrometric method to
measure the isotopomer ratios of N2O with high sensitivity and high precision in order to resolve the
complex N2O cycle and to provide a basis for reduction of anthropogenic N2O emission using
isotopes as an alternative tool (Yoshida & Toyoda, 2000).
3. Isotopic observations of N2O in various environment
We have been conducting isotopic observations of N2O in the atmosphere, ocean, land waters, soils,
and other sources. Monthly monitoring of surface air at Hateruma Island, Japan, showed that 15N/14N
ratio in atmospheric N2O has been decreasing since 1999. In the western North Pacific, dissolved
2012 Japanese-American Kavli Frontiers of Science
Ocean Acidification / N2O
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N2O showed a concentration maximum at about 500 m depth, and estimated
15
N/14N ratio and
18
O/16O ratio of N2O emitted from the ocean to the atmosphere are slightly larger than the values of
atmospheric N2O. In the eastern tropical North Pacific, where biological productivity is high because
of upwelling, the isotope ratios showed larger values suggesting that N2O is not only produced but
also consumed (reduced) by denitrifying bacteria.
Nitrous oxide emitted from fertilized agricultural soils showed significantly low
the
15
N/14N ratio reflecting
15
N/14N ratio of nitrogen fertilizer and microbiological isotope effect. From the characteristic
isotope ratios in N2O dissolved in river water in Tokyo, we found that significant amount of N2O is
emitted from wastewater treatment plant (WWTP). Detailed studies on N2O produced in WWTP
together with laboratory studies on pure culture of nitrogen-metabolizing bacteria revealed that the
N2O emission depends on type of biological treatments and that isotopomer ratios can be used to
specify the bacterial process that mainly contributes to the N2O production.
4. Analysis of global budget
Based on above-mentioned observational data including those reported by other researchers, we
analyze the global N2O budget using a mass balance of light and heavy isotopes between various
sources, atmosphere, ocean, and removal processes. The isotope budget calculation with
14
N-containing N2O suggested that the isotopically light (i.e.,
15
15
N- and
14
N/ N ratio is low) sources such as
agriculture and industry contribute to the increase of atmospheric N2O and that the contributing
source might have become heavier in recent years probably because of qualitative change in
anthropogenic sources.
Conclusion
Stable isotopes have been used to trace the cycle of water, carbon dioxide, methane, and so on. Our
study indicated that they can also be an invaluable tool for analysis of N2O cycle. Further studies on
temporal and spatial distributions of N2O isotopomers and isotopic characterization of various N2O
sources would enable (1) the global three-dimensional model simulations to understand the precise
picture of N2O cycle and (2) proposals of how to reduce N2O emission by human activities.
References
IPCC (Intergovernmental Panel on Climate Change) (2007), Climate Change 2007: The Physical
Science Basis., edited by S. Solomon et al., pp. 996, Cambridge Univ. Press, Cambridge, U.K.
Ravishankara, A. R., J. S. Daniel, and R. W. Portmann (2009), Nitrous Oxide (N2O): The dominant
ozone-depleting substance emitted in the 21st century, Science, 326, 123-125.
Yoshida, N., and S. Toyoda (2000), Constraining the atmospheric N2O budget from intramolecular site
preference in N2O isotopomers, Nature, 405, 330-334.
2012 Japanese-American Kavli Frontiers of Science
Ocean Acidification / N2O
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